1. Field of the Invention
The present invention relates to photovoltaic conversion devices represented by solar cells, and in particular, to a photovoltaic conversion device in which the properties thereof is improved by reducing influence of impurities mixing into during the manufacture thereof.
2. Description of the Related Art
In the specification, “aEn” represents “a×10n”.
Bulk photovoltaic conversion devices using multicrystalline silicon substrates now occupy a major part of the market of solar cells.
Generally, multicrystalline silicon substrates are manufactured by a casting method. However, a number of metal elements are included as impurities in the multicrystalline silicon substrates that are sliced out of a multicrystalline silicon ingot produced by the casting method.
Metals, in particular, transition metals are typical factors that degrade the properties of solar cells. Therefore, reducing the influence thereof is indispensable for improving the efficiency of solar cells.
Some typical transition metal impurities included in silicon substrates after casting and before they are processed into solar cells are Fe, Cr, Ni, and the like.
Although values of concentration of these impurities vary depending on the positions thereof inside the substrate because of the reason described later, they are about 1E11-1E13/cm3 when the total impurity concentration is measured by atomic absorption spectrometry, the ICP-MASS method, activation analysis, or the like. Meanwhile, the concentration thereof could be 1E14/cm3 or so in the extremely low quality peripheral regions of the substrate described later, which is not rare.
These values correspond to electrical qualities of about 33-0.33 sec in minority carrier life time τ, and about 320-32 μm in minority carrier diffusion length L, if 3E-14 cm2 is assumed as the capture cross section. Also, the values correspond to photovoltaic conversion efficiencies of about 12-16% under the conditions of the current structure and manufacture process of solar cells of our corporate.
In other words, in cases where minority carrier life time τ and minority carrier diffusion length L are distributed inside the substrate within the foregoing ranges, photovoltaic conversion efficiencies of 12-16% can be obtained.
By the way, when classified broadly, there are two types of metal “mixing sources” in the casting method.
One of the “mixing sources” (hereinafter referred to as “mixing source 1”) represents metal impurities included in the silicon starting material itself or components used during the casting such as the mold, mold release agent, crucible, heater, and the like. These are captured in the silicon crystal ingot according to the segregation coefficients of the respective elements during solidification of the molten silicon in the casting process. Since segregation coefficients of transition metals are about 1E-5, about one out of a hundred thousand of transition impurities captured in the molten liquid are captured in the crystalline silicon.
Since metal elements belonging to the “mixing source 1” are captured in the crystalline silicon due to a segregation phenomenon, they are distributed with an almost uniform concentration along a solid-liquid interface at the time of solidification, and distributed along the solidification direction so that they have higher concentration in the later solidification period, that is, as the solidification degree increases. For this reason, if an ingot obtained after casting is sliced in the direction perpendicular to the solidification direction into substrates, impurities belonging to the “mixing source 1” are distributed almost uniformly over the entire substrate area as compared to the impurity concentration distribution brought about by the “mixing source 2” described later.
The other one of the “mixing sources”, which are referred to as the “mixing source 2”, are impurities that are thermally diffused toward the interior of the ingot from the mold (mold release agent or the like) in contact with the ingot during a high temperature period between the solidification and cooling to room temperature of the ingot.
The doses of impurity diffusion of the “mixing source 2” are determined depending on the impurity concentration in the component in contact with the ingot, the range of temperatures, and period of time during which they are exposed to a high temperature range in which thermal diffusion is effectively caused (hereinafter referred to as “exposure temperatures” and exposure time, respectively). For example, normally, Fe concentration in a mold release agent in contact with an ingot is about 10 ppm or so. However, when the exposure temperatures are 400-1000° C. and exposure time is about 7 hr, Fe is diffused from the mold/ingot contact interface to the interior of the ingot to a depth of 10-20 mm with a high concentration of about 1E13-1E15/cm3.
That is, crystalline silicon grains in bottom and side portions of an ingot with which they are in contact have extremely low quality. For this reason, the quality of substrates is particularly deteriorated in regions originating from a bottom region of the ingot, and in end portions thereof corresponding to a side portion of the ingot.
It is known that, among the transition metals, in particular, interstitial Fe included in p-type crystalline silicon doped with boron (B) can be estimated based on the amount of diffusion length L decreased after heat treatment. (Document [1]: G. Zoth et al, J. Appl. Phys. 67(1990) p6764). Similarly, it is known that it can be estimated by light irradiation.
These estimations utilize the following dissociation reaction: thermal dissociation or photo dissociation:FeB pair+heat or light→Fe_i (interstitial site)+B (boron)
The relationship between a diffusion length before dissociation Lbf and a diffusion length after the dissociation Laf is expressed as follows:Diffusion length Lbf before dissociation≧diffusion length Laf after dissociation
Although we leave more description in detail to the document [1], the principles will be described as follows: since carrier recombination ability of Fe_i, which does not pair with B and is present in a state of an interstitial site, is superior to that of the FeB pair (Fe present in a interstitial site pairing with the dopant element B) under light radiation intensities of about 1E12-1E14 photon/cm3·sec in a common SPV device typically used for the linear SPV method, the diffusion length L decreases further in a forcibly, thermally dissociated or optically dissociated state, so that the concentration of the original FeB pair before dissociation can be estimated based on a variation of the diffusion length between before and after the dissociation.
If Fe present in an interstitial site is left at room temperature for a sufficient length of time, almost 100% thereof can be assumed to be present in a state where it pairs with B (FeB state). This is because B is included at a concentration of 1E16/cm3 or so in normal p-type crystalline silicon substrates for solar cells, which is sufficiently high with respect to Fe.
In addition, it is known that not only FeB pairs but also CrB pairs are dissociated by thermal dissociation. On the other hand, while FeB pairs are dissociated, CrB pairs are not dissociated by photo dissociation. Furthermore, it is known that while diffusion length decreases when FeB pairs are dissociated, diffusion length increases when CrB pairs are dissociated. Accordingly, photo dissociation is preferably used in cases where concentration of FeB pairs is desired to be determined with influence of increase in diffusion length due to dissociation of CrB pairs being previously eliminated. Therefore, hereinafter, the method for dissociating FeB pairs that is referred to in association with Fe concentration is assumed to be photo dissociation.
In normal multicrystalline silicon substrates fabricated by casting, concentrations of FeB pairs are about 1E11-1E12/cm3 in a state before they are processed into solar cells in central portions of the substrates excluding the bottom portion of the ingot and peripheral portions of substrates whose quality is degraded due to the “mixing source 2”, although the values depend on the silicon starting material and the quality of casting components.
In order to reduce adverse effects of metal impurities, what is important first is to reduce contamination from the doping sources of metal impurities. However, it is not necessarily easy. This is because significant purification of the starting material and components leads to higher costs, it is not technically easy, and a certain degree of impurity mixing is inevitable in the process.
Therefore, supposing that a certain degree of impurity mixing is inevitable, reducing influence of mixed impurities as much as possible is important. Related to this, some techniques have been developed.
Gettering is widely known as a typical technique of that kind. Gettering is a technique for transferring impurity elements from a device region, which mainly represents a light active region in the case of a solar cell, to another region where they are secured. A well known, typical example thereof is gettering effect of metal elements by P (phosphorous) diffusion. This is referred to as “P gettering.” In photovoltaic conversion devices according to the present invention, the P gettering effect is utilized when P is thermally diffused to form a pn junction for improvement of the properties.
The foregoing conventional P gettering effect that is additionally utilized for pn junction formation in photovoltaic conversion devices is not necessarily sufficient.
This is because the primary object of the P thermal diffusion process is to form a good pn junction, and such a condition for forming a good pn junction does not overlap a condition for maximizing the P gettering effect.
That is, a good condition for forming a good pn junction in normal photovoltaic conversion devices where the side of the pn junction is the light receiving surface requires not only achieving good diode characteristics, but also reducing light absorption loss caused by an n-layer that is highly concentrated as a result of P thermal diffusion as much as possible. Sufficient photovoltaic current for high efficiency solar cells cannot be obtained unless the thickness of the highly concentrated n-layer is made to be as small as 0.1-0.3 μm or less. In order to realize such a thin thermal diffusion region, for example, the peak temperature for thermal diffusion should be maintained in a range of about 800-850° C., and the time for the thermal diffusion should be about several minutes to 10 minutes.
However, this is not enough to extract a maximum P gettering effect.
To consider diffusion length of impurity elements that are the target of gettering as an index of gettering effect, it is about several tens to 200 μm for Fe and Cr under the foregoing P thermal diffusion conditions (thermal and temporal conditions). Since the thickness of a substrate is normally about 300 μm or so, from a view point of impurity diffusion length, gettering effect can be obtained advantageously to some extent.
However, in order to fully extract gettering effect, only considering the distance index is not sufficient.
While the concentration and thickness of the P thermal diffusion region are important indices that affect the gettering effect, it has been difficult to achieve sufficient values of them in conventional P thermal diffusion processes in which the peak temperature is restricted and the time for the thermal diffusion is short for forming a shallow pn junction.
In particular, because the P thermal diffusion process completes before gettering effect works sufficiently on substrates taken from a bottom region of the ingot where Fe concentration is high and peripheral regions of substrates originating from a side region of the ingot, despite a certain level of improvement in quality, the regions are still left as lower quality regions. As a result, sufficient improvement in properties cannot be achieved.
Here, the degree of insufficiency of P gettering can be estimated by comparing diffusion lengths Lafter returning the cell into the state of a substrate, that is, returning it into the state of a p-type silicon substrate by removing all of the electrodes, antireflection film, and junction layers, leaving only the bulk p-type region, which is a light active layer. In other words, if P gettering is sufficiently achieved in the entire substrate, diffusion length before light irradiation L≦diffusion length after light irradiation should be satisfied in the entire substrate. On the other hand, if it is insufficiently achieved, the ratios of the regions having the foregoing inequality relationship to the area of the substrate become smaller in parallel to the degree of insufficiency, by which estimation can be done.
For example, under the conventional P thermal diffusion conditions, since the foregoing sign of inequality relationship: diffusion length L before light irradiation≦diffusion length Lafter light irradiation is satisfied for the region of the substrate originating from the “mixing source 1”, it is possible to verify that sufficient gettering effect is achieved. However, in the region of lower quality with serious metal contamination originating from the “mixing source 2”, diffusion length L before light irradiation>diffusion length Lafter light irradiation is satisfied. That is, there are quite a few areas where FeB pairs are present at levels that are measurable by the SPV method. Therefore, this region with insufficiently improved quality is a great factor that restricts improvement of properties.
Meanwhile, improving the quality of substrates is relatively easy if the bottom portion of the ingot or side thereof that constitute the “mixing source 2” are removed. However, in this case, removal should be done referring to the above described thermal diffusion distance of about 10-20 mm. If this part is disposed of as waste, the part of silicon material that is disposed of leads to an increase of cost, which is problematic.
As discussed so far, it is obvious that if the region of the ingot with low quality originating from the “mixing origin 2” is sufficiently removed, improvement in properties can be achieved. Therefore, a method which allows the region of the ingot with low quality to be as free as possible from disposal as a result of trade-off between quality and cost has been anticipated.
A primary object of the present invention is to provide a photovoltaic conversion device using a crystalline silicon substrate with high electric properties in which influence of impurities such as Fe is reduced, and a process for manufacturing the device.